This invention relates to snubber and clipper circuits for limiting the voltage across a power transistor.
FIG. 1 illustrates a portion of a prior art single-ended forward converter 10 constructed in accordance with the prior art. Referring to FIG. 1, an input voltage V.sub.in is applied to an input terminal 12, which in turn is coupled to the primary winding 14 of a transformer 16. Periodically, a control circuit 18 turns on a transistor 20, thereby causing current to flow from terminal 12, through winding 14 and transistor 20, and to ground, and causing current to flow through a secondary winding 22, through a diode 23 and a choke inductor 24, and to load circuitry comprising a filter capacitor 26 and a load 28. In order to modulate output voltage V.sub.out at output lead 30, periodically, control circuit 18 turns off transistor 20 so that power is no longer transferred from primary winding 14 to secondary winding 22. (During the time period in which transistor 20 is off, choke inductor 24 typically continues to draw current through a diode 31.) By controlling the duty cycle of transistor 20, the amount of time power is delivered to winding 22 from winding 14, and the amount of energy delivered to load 28 each switching cycle can be controlled. Further details concerning single-ended forward converters (and other types of power supplies) are described in "High-Frequency Switching Power Supplies: Theory and Design" by George Chryssis, published by McGraw Hill Book Company in 1984, incorporated herein by reference.
Because of the inductive nature of transformer 16, when transistor 20 is on and current is flowing through windings 14 and 22, magnetic flux, corresponding to the volt-seconds applied across winding 14, accumulates in transformer 16. Accordingly, when transistor 20 turns off, a high flyback voltage appears at lead 14a of winding 14. If the flyback voltage at lead 14a is sufficiently large and exceeds the maximum voltage specified by the transistor manufacturer, it could cause a catastrophic failure in transistor 20. Accordingly, it is known in the art to provide snubber circuits across transistors to limit the voltage excursions across the transistors and prevent their destruction. Unfortunately, prior art snubber circuits dissipate power, thereby reducing the efficiency of the power supply.